1. Field of the Invention
The present invention relates to inverter circuits, especially to inverter circuits used in uninterruptible power supplies.
2. Description of the Prior Art
The need for high quality, reliable, inexpensive uninterruptible power supplies is becoming much more critical as the use of computer systems and other relatively sensitive electronic equipment increases. Uninterruptible power supplies are used to provide continuous alternating current power to the attached equipment so that potentialy catastrophic failures do not result should the general alternating current power supply be removed or should the genera alternating current power supply contain undesirable transients. Uninterruptibe power supplies generally include rectifier circuits, a battery, a battery voltage booster circuit and an inverter circuit. The inverter circuit is used to provide an alternating current output voltage acceptable for use by the attached equipment from a direct current input voltage.
The outputs of most uninterruptible power supplies are either square wave or sine wave voltages. However, because most electronic systems are designed to operate on the specific root mean square and peak value input waveforms found in sine wave sources, the square wave output supplies do not power the systems in a predictable fashion and are therefore not sufficiently reliable without extended testing. Thus, sine wave outputs are preferable for production by uninterruptible power supplies.
Inverters are the circuits which produce an alternating current waveform from a direct current voltage source. Typically, inverters used transistors to control the voltage and current to a series of inductors and capacitors to develop the alternating current supplied to the load. A reference waveform was sometimes developed for transistor control purposes.
There were a number of common circuit designs used to form inverters. One common circuit used in an inverter used four transistors in a bridge configuration and a monopolar direct current voltage supply. The direct current voltage source was connected to two opposite corners of the bridge, while the load was connected in series with switching inductors between the other two corners of the bridge. Switching capacitors were generally connected from the load terminals to the direct current ground voltage. A pulse width modulated drive signal was supplied lo the transistors to control the conduction of opposinq transistors in the bridge and an alternating current was developed by alternatingly changing the conduction of the transistor pairs. This circuit design had the general disadvantage that only one transistor in each pair could be driven by a ground referenced control signal, while the other transistor had to be driven by a control signal which was referenced from the direct current positive voltage. This control signal referenced from the positive supply was difficult to develop and complicated the inverter circuits of this class.
A second common inverter circuit used to create a sine wave output required a bipolar direct current voltage source. In that design only two transistors were needed, each transistor coupled to one of the direct current voltages and the two transistor outputs coupled to each other. From this common point a switching inductor and the load were connected in series to the ground voltage, with a switching capacitor connected from the load and inductor junction to the ground voltage. The transistors were alternately activated so that the switching inductor was alternately coupled to the positive and negative direct current voltages to develop an alternating current waveform at the load. The circuit had the disadvantage that it required the use of or development of bipolar direct current voltages, with the additional capacitors and diodes needed over a monopolar direct current system and resulted in increased complexity in battery boost voltage circuitry when used in an uninterruptible power supply application.
It is desirable to have a circuit which allows transistors which are controlled by a ground referenced siqnal for ease in developing the transistor control signal and which allows the use of a monopolar direct current voltage source. Additionally, it is desirable that the circuit operate at its own duty cycle of transistor switching operation and activate a transistor only when the output voltage waveform is outside desired limits.
U.S. Pat. No. 4,553,039 disclosed an uninterruptible power supply utilizinq a center tapped transformer with inductors and capacitors as energy storage devices. This patent disclosed a two transistor switch design with both transistor switches ground referenced. Additionally, this patent disclosed the use of voltage feedback from the primary or input winding of an output transformer and a circuit which had a self-determined duty cycle. The output waveform reference used in the circuit disclosed in the patent was developed by the use of two separate reference waveforms, one for the upper half cycle of the desired sine wave output and one for the lower half cycle of the desired sine wave output. The upper half cycle reference waveform was coupled to one comparator circuit while the lower half was coupled to a second comparator circuit, with one transistor switch controlled by each comparator. The comparators were configured so that if the voltage was respectively too far below the upper half cycle reference or too far above the lower half cycle reference, the appropriate transistor switch was energized to increase the magnitude of the output voltage. This drive system worked in some situations, but the circuit would not provide a proper output waveform should the magnitude of the voltage be too great, for example, more positive than the positive reference waveform or more negative than the negative reference waveform. The design disclosed would not bring the output voltage back to within a desired tolerance range. Additionally, if the two reference waveforms were not identical a non-uniform output waveform developed, with resulting problems.
U.S. Pat. No. 4,489,371 disclosed a four active device bridge circuit utilizing a self-determining duty cycle feedback control circuit. The output voltage of a transformer was fed back and used by a comparator which had a desired reference voltage. The comparator circuitry was designed so that one or the other of the active device pairs used in the design were activated at all times. Because one of the active device pairs was activated at all times the voltage output was always hunting or oscillating about the desired reference.